Typical larger burner systems have a combustion air duct through which air is drawn to a combustion chamber by an electrically powered blower. The combustion chamber houses a fuel injector or nozzle which provides fuel to the chamber from a source such as a tank or a gas main. Flow of fuel to the combustion chamber is controlled by an electrically operated fuel valve which is in turn opened and closed responsive to a control signal. The fuel is typically ignited by an igniter which also receives its own control signal. A control system provides the control signals to the valve and the igniter and other elements of the system as well according to a prearranged operation sequence which schedules the order and time of each activity in the burner system. The schedule is established in the factory either by the setting of timer elements or if the controller is microprocessor-based, by factory installed software. A demand switch which controls an input to the control system starts the operation sequence when it closes. Typically, the demand switch is a thermostat which either directly controls current flow to the burner system, or actuates the winding of a relay whose contacts control the current flow.
It is necessary for safety that ignition not be attempted until the combustion chamber has been purged of any residual combustible gasses. To accomplish this, the control system in following the operation sequence first of all starts the blower and allows it to run for a period of time sufficient to assure a number of air changes within the combustion chamber and eliminate the possibility of these residual gasses.
Because of the importance of adequate purging before ignition is attempted it is now the practice to insert an airflow switch to sense flow of air in the combustion air duct. The airflow switch in one embodiment is actuated by the differential pressure created by the moving air stream. Another type of switch is nothing more than a simple normally open ON-OFF switch connected to a small sail which in response to the pressure of moving air causes the airflow switch to close. The only test which most systems currently in use now make as to proper function of the airflow switch is that the switch is closed during purging and running. Other systems sense whether the airflow switch is closed at the time the demand switch is closed, and if so aborts the startup sequence. These systems treat a malfunction resulting in such an aborted startup as one requiring an operator to intervene.
Because of the importance of proper air flow, those responsible for safe design of burner systems are now coming to the conclusion that it is important to sense that the airflow switch is open each time when the operating sequence is initiated, and that it closes at an instant shortly after the start of the operating sequence. While normally these switches are quite reliable, it is possible that defective installation or maintenance may leave the airflow switch permanently closed. Those systems which sense only that the airflow switch is closed after the startup sequence is initiated cannot sense such a condition. The most recent system designs resolve this problem by completely shutting down the startup sequence and requiring manual intervention to reset the system for another startup sequence. There is however, one situation can arise where manual intervention is unnecessary. This is where airflow switch failures are only temporary, and the switch will after a few seconds function properly again.
Such a "failure" arises when there is a power outage. A power outage removes power from the fuel valve because the designs of these valves are always such that when power is removed, the valve will close and combustion will cease immediately. Because the demand switch will still be closed, the startup sequence will be immediately rerun when power is restored. It is likely the airflow switch will remain closed after the start of the outage because the momentum of the blower impeller and motor is sufficient to maintain substantial air flow through the duct for a time. Thus, when the operation sequence restarts, if the outage is sufficiently short the closed airflow switch will be treated by systems which now sense the initial condition of the airflow switch, as a malfunction requiring operator intervention. This of course is a real irritant if the "fault" is caused by a momentary power outage for which no operator intervention is necessary.
Accordingly, a control system which merely suspends execution of the startup sequence when the airflow switch is detected as closed at the start of the operation schedule, and upon opening of the airflow switch permits normal startup, will improve the convenience of such systems.
There are a number of references which pertain to management of air flow to a combustion chamber. Perhaps the most relevant of these is U.S. Pat. No. 4,403,942 (Copenhaver) which describes a system for suspending operation if the airflow switch is closed when the demand switch closes. This system is apparently for use with relatively small furnaces of the type simultaneously enabling the blower, fuel, and ignition. Other references which provide background art include U.S. Pat. Nos. 3,263,731 (Hanna et al.); 4,412,328 (Homa); 4,451,226 and 4,451,225 (Landis et al.); 4,695,246 (Beilfuss et al.); 4,5l8,245 (Mueller et al.); and 4,792,089 (Ballard).